In the semiconductor industry, particularly in the segment of the semiconductor industry focused on communication applications, gold is widely used as a conductive material. When gold is used to form conductive features, a thin layer of gold is frequently deposited and employed as a seed layer. Subsequently, for example, after electrolytic deposition, certain portions of the gold seed layer are no longer desired and thus need to be removed from the semiconductor workpiece.
Both wet-etching and electrolytic etching can be used to remove a gold seed layer from semiconductor workpieces. One wet-etching process is disclosed in U.S. Pat. No. 5,221,421 to Leibovitz et al. One disadvantage associated with a wet-etch process is that it can produce levels of surface roughness on the gold features that are considered undesirable by manufacturers of semiconductor devices. Another disadvantage of a wet-etch process is that it results in undercutting around the base of the gold features. Undercutting of the gold features is undesirable because it compromises the mechanical strength and electrical properties of the features. In addition, the wet-etching process conditions need to be strictly controlled. For example, small variations in temperature and/or reagent concentration significantly affect the amount of gold removed. This problem may result in over-removal and over-undercutting.
To this end, an electrolytic process is easier to control and is advantageous over a wet-etching process. Both thiourea and cyanide have been used in commercial baths and processes for electrolytically etching gold from a semiconductor workpiece. The problem associated with the process using thiourea is that thiourea is a suspected human carcinogen. Cyanide is a very poisonous chemical exposure to which harms the brain and heart. Thiourea and cyanide pose potential health and safety risks in the workplace. Moreover, disposal of a thiourea-containing and cyanide-containing bath presents an environmental hazard.
Accordingly, the primary advantage of the present invention is that it electrolytically removes gold without the use of thiourea or cyanide.